Precambrian Earth and Life History Notes

Precambrian Earth and Life History

Hadean eon

What Happened During the Hadean?

  • Zircon = oldest mineral = 4.4 byo - may confirm oldest continental crust, after the Earth-moon system was formed

  • Evidence of crust during the Hadean includes:

    • Metamorphic rocks = 4.28 billion years old

    • 3.96 billion years old rocks from Northwest Territories, Canada, reported earlier.

Shields, Platforms, and Cratons

  • Shields: Large areas of exposed Precambrian rock (Canadian shield)

  • Platforms: Precambrian rocks covered with younger rocks.

  • Craton: shield + platform combo = forms stable continental core

Hadean - Very Young Earth

  • Shortly after Earth formed, it was very hot due to:

    • Colliding particles during Earth's accretion (coming together by gravity)

    • Gravitational compression.

    • Decay of short-lived radioactive isotopes.

  • The young Earth in the Hadean was very different compared to later Precambrian Earth:

    • A relatively high frequency of meteorite and comet impacts.

    • Lethal doses of ultraviolet radiation due to the absence of an ozone layer.

    • The Moon was much closer to Earth, resulting in a day length of only 10 hours.

Formation of Continents

  • magma sea cooled = crust

  • continents formed from chemical differences in magma

Archean eon

Archean - Growth of Early Continents

  • Island arcs added to the continent crust - subducting ocean crust

  • Sediments extend continental materials seaward.

  • Continent-Continent collisions resulted in larger continents.

  • Magmatism from subduction zones causes thickening.

Proterozoic eon

Proterozoic Earth History

  • Major difference in crustal evolution style:

    • Passive margin sediments were common.

    • Crust accreted around continental nuclei to create large cratons like Laurentia.

Proterozoic Supercontinent

  • supercontinent Columbia = 2.1 to 1.8 bya (started fragmenting around 1.5 to 1.35 billion years ago) - also known as Nuna and Hudsonland - had parts (proto-cratons) making up the core of other supercontinents

  • Rodinia: 1.1 byo, Laurentia/Australia/Antarctica collided

Proterozoic Deposits

  • Thick sedimentary packages of sand, shale, and carbonates formed on passive margins during the Proterozoic.

  • Ophiolite sequences: stack of ocean floor crust that got pushed onto land - was the first observed difference of oceanic and continental crust

Proterozoic Glaciations & Snowball Earth

  • Major glaciations occurred in the Proterozoic:

  • 👉 Huronian = early/longest ice age - linked to great oxygenation event (GOE) - increased oxygen, decreased methane = cold - south of Hudson Bay
    👉 Cryogenian = extreme global ice (Snowball Earth):

→ 5 stages: oceans start freezing, “snowball earth” - lowered reflectivity, CO2 outgassed by volcano, greenhouse effect melts “hothouse earth”, CO2 pulls back into oceans - back to normal

Origin and Evolution of Atmosphere and Oceans

  • Early atmosphere formed from volcanic outgassing, primarily water vapour and CO2

  • Early atmospheric conditions were not suitable for modern oxygen-breathing organisms until photosynthetic organisms (Eubacteria) evolved, approximately 3.5 billion years ago.

  • most oxygen = cyanobacteria in stromatolites - 2.3 bya

Great Oxidation Event (GOE)

  • significant increase in atmospheric oxygen approximately 2.5 to 2.1 billion years ago.

  • Led to the formation of banded iron formations (BIF)

Life - Its Origin

  • Earliest sign of life = 3.5 billion years ago, primarily fossil evidence of microorganisms (cyanobacteria)

  • Archean: biologically impoverished compared to present - no oxygen

  • living organism: must reproduce and have metabolism

  • two requirements for origin of life = elements that make up organic molecules, and energy sources

Organic Chemical Evolution on Earth

  • A. Z. Oparin hypothesized that chemical evolution preceded cellular life formation, suggesting spontaneous generation of organic molecules like amino acids under early Earth conditions.

  • Stanley Miller's experiments confirmed that diverse organic compounds could arise through realistic primordial conditions.- gases used = H2, H2O, ammonia and methane

Origin of Life

  • required spontaneous organization of self-replicating organic molecules, the requirements:

  1. membrane-enclosed capsule

  2. energy-capturing chemical reactions

  3. chemical system for replication (RNA-DNA)

Fossil Evidence of the Origin of life

  • trace of life: stromatolites (organic structure) - 3.5 bya - cyanobacteria - Archean

  • sexually reproducing cells appeared mid Proterozoic - new organisms developed

  • gunflint microfossils: 2 bya

  • euk. cells = early/middle Proterozoic

  • oldest eukaryotes = grypania (photosynthetic algae)

Ediacara Fauna

  • impressions of multicellular, soft-bodied animals in rocks - 600 mya

Late Proterozoic Shelly Fossils

  • shells/shell-like material in rocks → Cloudina = oldest shelly animal fossil

—> 60% of Manitoba = underlain by Precambrian rocks